1. Field of the Invention
The present invention relates to a magnetic material formed by magnetically coupling a ferromagnetic material and an antiferromagnetic material, a method of producing the same, and a magnetic recording medium using the magnetic material.
2. Description of the Related Art
As magnetic recording media, magnetic tape formed by coating magnetic particles on a substrate, floppy discs, and magnetic disc devices enabling random access as external storage devices of computers, that is, so-called “hard discs”, are being widely used.
A magnetic disc is comprised of a substrate formed on one surface with a layer-like recording medium comprised of a magnetic material. Fine magnetic particles are filled in the thin film recording medium uniformly at a high density and in a good dispersion state. A magnetic head moving above the recording medium along a predetermined track magnetizes a group of fine magnetic particles of the magnetic material corresponding to one bit or determines a magnetization state of the group of magnetic particles to record or reproduce one bit of data.
One of the methods for raising the recording density of a magnetic disc device is reducing the thickness of the recording medium and increasing the fineness of the magnetic particles of the magnetic material forming the recording medium.
When magnetic particles of a magnetic material are increased in fineness, however, the effect of so-called “thermal fluctuaction” appears. If the effect of the thermal fluctuaction exerted upon the magnetization becomes large, the direction of the recorded magnetization of the fine magnetic particles is reversed by the surrounding thermal energy and the direction of the magnetization lost, or the residual magnetization or reproduction output declines along with the elapse of time. Due to this, the magnetic recording medium gradually becomes unable to stably maintain a recorded state over a long time.
The stability of a magnetic particle against the thermal fluctuaction may be expressed byKuV/kT
where,
Ku is the magnetic anisotropy energy per volume of a magnetic particle,
V is the volume of a magnetic particle,
k is the Boltzmann constant, and
T is the absolute temperature.
The smaller the value of KuV/kT, the larger the influence of the thermal fluctuaction. The higher the recording density, the smaller the volume V of a magnetic particle, so the smaller the KuV/kT and the weaker the resistance to the thermal fluctuaction.
The smaller the volume V of a magnetic particle, the smaller the magnetic anisotropy energy of the magnetic particle and the shallower the potential energy barrier for stabilizing the magnetization direction, so the magnetization vector of the magnetic particles easily escapes from that potential energy barrier even due to the energy of the thermal fluctuaction and the magnetization state becomes unstable.
The magnetic field necessary for changing the magnetization direction of a magnetized magnetic particle is referred to as the “coercive force”. If the coercive force of the magnetic recording medium is low, the magnetization state of the magnetic particles will change and become unstable even by a small external effect such as the thermal fluctuaction. Conversely, if the coercive force is high, the magnetization state will be difficult to change by the thermal fluctuaction, so resistance to the thermal fluctuaction will be strong and the stability of the recorded state can be secured.
Accordingly, in order to achieve high density recording, it is necessary to increase the magnetic anisotropy energy of the magnetic particles, and consequently raise the coercive force of the magnetic material and overcome the influence of the thermal fluctuaction.
Further, the magnetic material forming the magnetic recording medium is being required to be more excellent in corrosion resistance, smoothness, abrasion resistance, the ability to secure a low process temperature, and various other characteristics relating to ease of production and convenience in usage. However, it is not easy to simultaneously satisfy the requirements of magnetic anisotrophy energy, coercive force, and the above characteristics.
One method being experimented with to increase the magnetic anisotrophy energy of a magnetic material to raise the coercive force is the method of utilizing magnetic coupling of a ferromagnetic material and an antiferromagnetic material. Such a magnetic material is attracting attention as a magnetic material able to provide a large magnetic anisotrophy energy and a high coercive force while largely reducing the thickness of the ferromagnetic thin film.
For example, the thin film-shaped magnetic material disclosed in Japanese Unexamined Patent Publication (Kokai) No. 11-296832 utilizes magnetic coupling of a ferromagnetic material and antiferromagnetic material to raise the magnetic anisotrophy energy and the coercive force while sufficiently satisfying the reduction of the thickness of the recording medium and is therefore suitable for a high recording density.
FIG. 1 shows the configuration of a magnetic recording medium using such a thin film-shaped magnetic material.
The magnetic recording medium shown in FIG. 1 is comprised of a substrate 101 on one surface of which an antiferromagnetic layer 102 and a ferromagnetic recording layer 103 are stacked. The antiferromagnetic layer 102 acts as an underlying layer of the ferromagnetic recording layer 103. A nonmagnetic underlying layer or the like may also be formed between the substrate 101 and the antiferromagnetic layer 102.
Such a foil-like magnetic recording medium has a high anisotrophy energy and coercive force, so is suitable for reduction of thickness of the recording medium and a high recording density.
Turning now to the problem to be solved by the invention, to deal with future advances in magnetic recording devices, magnetic materials and magnetic recording media having more excellent properties are being demanded.
A magnetic fine particle having an antiferromagnetic phase and a ferromagnetic phase is one leading candidate. Such a magnetic fine particle would enable further fineness of the magnetic material, would enable a good dispersion state of the magnetic material to be secured, and could be expected to overcome the problem of the thermal fluctuaction and raise the coercive force.
The magnetic characteristics of a magnetic particle having an antiferromagnetic phase and a ferromagnetic phase, for example, a ferromagnetic particle having an antiferromagnetic shell, have been reported in W. H. Meiklejohn and C. P. Bean, Phys. Rev., vol. 102 (1956) 1413 and Phys. Rev., vol. 105 (1957) 904.
However, the phenomena reported there were observed in the case of cooling to a low temperature of 77 K, so the Co particles with CoO shells used there are not suitable for use for a magnetic recording medium at room temperature. Also, the reports dealt with “asymmetry of magnetization hysteresis curves” as the effect of the antiferromagnetic shell and not an increase of the coercive force important for magnetic recording.
As the material of fine magnetic particles having a sufficient coercive force, needle-like fine iron particles and barium ferrite fine particles having been considered to be candidates.
Iron-based metal particles, however, increase in surface area along with increased fineness and suffer from a severe problem of corrosion. Also, a barium ferrite fine particle has a plate-like shape, so they solidify by superposing the fine particles, therefore, no method has been established for realizing a good dispersion state for coating as a medium.